In recent years, use of a so-called resistance variable element (ReRAM element) as a memory element has been proposed. Such a resistance variable element has a resistance variable layer formed mostly of a metal oxide. Upon application of electric pulses to the resistance variable layer, its resistance values change and the changed resistance values are retained. By corresponding a high-resistance state and a low-resistance state of the resistance variable layer to data “1” and data “0” of binary data, respectively, for example, the binary data can be stored. The electric pulses are applied so that an electric field between both ends of the resistance variable layer and a current density at end surfaces of the resistance variable layer are generated to sufficiently change a physical state of the resistance variable layer and not to destroy the resistance variable layer. The electric pulse may be applied several times.
As a memory device using the above resistance variable element, a cross-point memory structure is known. In this structure, a memory plug is formed at a location where upper and lower wires three-dimensionally cross each other, and a resistance variable layer having a bipolar characteristic and a diode are arranged in series in a stack direction, inside the memory plug, thereby achieving high-dense integration and suppressing a cross talk (e.g., see patent literature 1).
From recent study results relating to the resistance variable element, it is found out that by forming the resistance variable layer consisting of a single layer conventionally as having a layer-stacked structure including two resistance variable layers, a favorable characteristic as the resistance variable element is achieved.
For example, non-patent literature 1 discloses that an insulator layer comprising an oxide and a metal resistance variable layer being electrically conductive are stacked together to form a resistance variable layer. In such a configuration, by controlling a thickness of the insulator layer, a so-called forming step (step of applying an electric stress to a resistance variable element after a step of layer stacking completes to enable the resistance variable element to switch its resistance state) may be dispensed with. In addition, a value of a current flowing through the resistance variable element can be controlled.
Patent literature 2 discloses a configuration of the resistance variable layer in which a first tantalum oxide layer having a composition expressed as TaOx (0<x<2.5) and a second tantalum oxide layer having a composition expressed as TaOy (x<y) are stacked together. In such a configuration, the forming step is unnecessary. Further, a high-speed, reversible and stable rewrite characteristic and a good resistance value retention characteristic are attained.